The invention relates generally to the delivery of polishing agents in a wafer processing system.
In the process of fabricating modern semiconductor integrated circuits (ICs), it is necessary to form various material layers and structures over previously-formed layers and structures. However, the prior formations often leave the top surface topography of an in-process wafer highly irregular, with bumps, areas of unequal elevation, troughs, trenches and/or other surface irregularities. Such irregularities cause problems when forming the next layer. For example, when printing a photolithographic pattern having small geometries over previously-formed layers, a very shallow depth of focus is required. Accordingly, it becomes essential to have a flat and planar surface. Otherwise, some parts of the pattern will be in focus and others will not. Surface variations on the order of less than 1,000 angstroms (D) over a 25xc3x9725 millimeter (mm) exposure area are preferred. Additionally, if the irregularities are not leveled at each major processing step, the surface topography of the wafer can become even more irregular, causing further problems as the layers stack up during further processing. Depending on the die type and the size of the geometries involved, the surface irregularities can lead to poor yield and device performance. Consequently, it is desirable to planarize, or level, the IC structures.
One technique for planarizing the surface of a wafer is chemical mechanical polishing (CMP). In general, CMP planarization involves holding a thin flat semiconductor wafer against a rotating wetted polishing surface, such as a compliant polishing pad, under a controlled downward pressure. During the CMP process, a slurry is provided to facilitate the polishing process. The slurry can result in higher removal rates of material from the surface being polished, as well as higher selectivity between various films and layers on the semiconductor substrate.
Typically, slurries used in CMP surface treatments include various constituents such as potassium hydroxide, silicon oxide, an oxidizer such as hydrogen peroxide, and a diluent such as de-ionized water. Other materials, such as ferric nitrate or chloride may also be present.
In the past, premixed bulk slurries have been used for CMP techniques. Unfortunately, some premixed slurries tend to gel quickly. As a result, such slurries can foul up the polishing system and lead to unevenly polished surfaces. Only a limited range of premixed slurries are available because of the balance required between the chemical stability of the slurry and the ability of the slurry to polish specified materials on the semiconductor substrate. Furthermore, the use of premixed slurries precludes dynamically changing the composition of the slurry.
In contrast to premixed slurries, slurry delivery systems have been proposed to deliver and mix the various slurry constituents just prior to the actual polishing. Unfortunately, some of the systems do not allow for sufficiently high precision and control with respect to the duration, amount and rate at which each slurry constituent is provided. A related problem encountered in some slurry delivery systems is that it is difficult to operate the system to dispense the slurry at a constant rate.
In general, according to one aspect, a method of providing a polishing agent to a wafer processing station includes operating multiple motor-driven pistons automatically to dispense respective amounts of respective polishing agent constituents from respective containers. The dispensed polishing agent constituents are delivered to the wafer processing station.
As used herein, a xe2x80x9cpolishing agentxe2x80x9d may include one or more fluids that are used during a CMP or other wafer polishing or buffing process. Examples of polishing agents include, for example, slurries, buffing agents, and cleansing agents, among others. A polishing agent may include one or more constituents.
According to another aspect, a polishing agent delivery apparatus for a wafer processing system includes multiple tanks each of which can hold a supply of a respective polishing agent constituent. The apparatus includes multiple containers, each one of which can hold a respective polishing agent constituent. Additionally, each container has a motor-driven piston that can be operated to cause the respective polishing agent constituent to be dispensed for delivery from the container to a wafer processing station and which can be operated to cause additional polishing agent constituent to be drawn into the container from a corresponding one of the tanks. The apparatus also includes a controller for controlling a respective rate of linear displacement of each motor-driven piston.
Various implementations may include one or more of the following features. The polishing agent delivery apparatus can include multiple stepper motors or servo-motors coupled to the controller. Each stepper motor can be coupled to a respective one of the pistons by a lead screw to control linear displacement of the piston to which it is coupled. Optical sensors can be used to provide signals indicative of the position of an associated one of the lead screws.
A sliding seal can be provided about the periphery of each piston. In some implementations, each container includes a diaphragm disposed between the piston and an end of the container through which the respective polishing agent constituent is dispensed. Each container also can include a hydraulic fluid disposed between its associated diaphragm and piston. The diaphragm can comprise an elostomer.
A first group of flexible hoses can be provided to connect the tanks to respective ones of the containers. A second set of flexible hoses can connect the containers to a delivery point adjacent the wafer processing station. Pinch valves or other valves which can be operated in open and closed states can be used to control flow of a polishing agent constituent between a respective one of the containers and associated ones of the flexible hoses. Air cylinders can be provided to control the states of the respective valves.
In some implementations, the pistons are operated to fill each container with approximately an amount of a respective polishing agent constituent substantially equal to the amount used during a single polishing cycle. The containers and hoses can be flushed with a cleansing fluid following one or more polishing cycles.
In yet another aspect, an apparatus for delivering a polishing agent to wafer processing stations in a wafer processing system includes multiple tanks each of which stores a supply of a polishing agent constituent. The apparatus also includes multiple groups of containers. Each container includes a motor-driven piston that can be operated to cause a respective polishing agent constituent to be dispensed for delivery from the container to a respective one of the wafer processing stations associated with the group of containers to which the container belongs. Each container can be operated to cause additional polishing agent constituent to be drawn into the container from a corresponding one of the tanks. The apparatus also has a controller for controlling a respective rate of linear displacement of each motor-driven piston.
Various implementations include one or more of the following advantages. The system allows changes to slurry mixtures or other polishing agents to be made quickly and efficiently, and different slurry or buffing mixtures can be used for different wafer processing stations. The rates at which one or more polishing agent constituents are dispensed can be altered dynamically from one wafer processing cycle to another, as well as during a wafer processing cycle. Furthermore, the system can provide tight control over the dispensing operations so that precise, predetermined amounts of each constituent are mixed into the slurry or other polishing agent. The system can be made highly modular to facilitate the delivery of slurry to additional polishing stations.
Other features and advantages will be apparent from the detailed description, the drawings and the claims.